Winter tyres=Winter tyres only have to save you from tragedy once to be a worthwhile investment,

Did you know that below 7˚C, the rubber molecules in your summer-spec tyre treads progressively harden and freeze? This means the tyre is less able to adapt to the contour of the road surface, which in turn reduces grip levels, to such an extent that when braking on a wet – not icy – road from 50mph, summer tyres will need 40 metres to stop, when winter tyres will stop in 34 metres.

It’s a big difference and it’s surprising how few people know that there is such a fall-off in performance of the tyres we normally buy, just because the weather is colder. Of course, the difference on ice and snow is even greater, as the winter tyres’ design cuts through snow and into ice to give a far greater margin of safety than on normal tyres: braking on icy roads from 20mph, summer tyres take 68 metres to stop, while winter tyres will bring you to a halt in 57 metres.

In northern or mountainous regions of Europe where locals are often required to fit winter tyres, the fact that yours is on summer tyres will make insurance companies blame you first for any accident you’re caught up in. Germany requires drivers to swap tyres with the seasons unless using ‘all-season’ tyres.

Of course, it’s wrong to look at these comparisons in isolation: there are many more influences on tyre performance. Pressure is one. Manufacturers used to recommend that winter tyres were inflated to slightly higher pressures than their summer equivalents, on the grounds that winter tyres don’t heat up as much in the cold weather, but Robin Winter (yes, really...) of Vredestein says that the advice now is to maintain normal pressures.

Tread depth also affects performance. The difference between a new tyre and one worn but just legal is even greater than the wet road comparison above.

The age of the tyres can have an even greater effect, partly because modern compounds are around 30% better in braking than they were 25 years ago, but also because age hardens tyre compounds and makes them less supple and hence less grippy. There’s increasing evidence that we really should change our tyres once they’re 10 years old, whatever their wear levels. How do you know the age of your tyres? Use the checklist below…

So why not use winter tyres all year round? The simple answer is that at higher temperatures, on wet or dry roads, they’re not as good as summer tyres: tyre technology has moved forward a long way but temperature still has a big impact on longevity and performance.

As a result, we have summer tyres, winter tyres and all-season tyres, the last a compromise for all-year-round use but not matching the ultimate performance of either winter- or summer-specific tyres.

Winter tyres use a higher proportion of natural rubber, with silica compounds to maintain flexibility in cold conditions. Michelin’s Alpin range (made in Dundee, Scotland) even use a compound containing sunflower oil for better wet grip. Winter tyres also have far more sipes for cutting through water and deeper tread than summer tyres but the side effect is that they can make the steering feel imprecise in the dry; much effort has gone into encouraging the sipes to ‘lock’ when cornering, to reduce this effect.

With current tyre fitting costs, the most cost-effective way to use winter tyres is to have two sets of wheels (usually steel for winter, since alloy corrodes so badly from road salt) and swap summer for winter in October, then back in April. About 25% of motorists on the European continent do exactly that. If you use your car all year round, the arguments for changing to winter tyres when the temperature drops below 7˚C are compelling.How old are my tyres?The DOT code, which is required for the American market, can tell you when the tyre was made.

If it ends with three numbers, it was made in the 1980s; if it ends with three numbers followed by a triangle, 1990s; if it’s a four-number code, 2000s.

Within the code, the first two numbers are the week of the year, followed by the year of manufacture (single-digit 1980s/’90s, two since). So, 256 is the 25th week of 1986; 256 is the 25th week of 1996, 2506 is the 25th week of 2006.

If there’s no DOT code, either the tyres were not offered on the US market or they are over 25 years old… The two letters in the DOT code tell you the factory/country in which the tyres were made: see www.carbibles.com/tyremanufacturercodes.html.

What else can my tyres tell me?M+S tyres are for Mud and Snow; if they also have a very chunky tread pattern, they’re primarily for off-road use, but M+S-marked tyres that are primarily on-road patterns are increasingly common. Winter tyres that are marked with a snowflake symbol have passed an industry standard braking test on snow.

All tyres sold in Europe after July 1997, unless they’re for pre-1939 cars, must be E-marked (though there is no law to prevent you fitting non E-marked tyres to your car). The code tells you the type approval certificate, the country in which it was issued (‘11’ in a circle is UK) and the European requirement it complies with.

A yellow (usually) dot on the sidewall indicates the (fractionally) lightest spot on the tyre – it’s impossible to get them absolutely 100% uniform. This should be mounted adjacent to the valve to minimise the need for balance weights. A red dot indicates the highest spot (it’s also impossible to get them absolutely 100% round). If the wheel rim has a dimple in it, that marks its lowest spot, so the two should be mounted adjacent to one another (this takes preference over the yellow dot).

Radial coloured stripes in the tread indicate a tendency for the tyre to pull slightly to one side. If the stripe is near the centre, it will have little pull. If it’s further out, try to mount that tyre opposite one that has its band to the other side, so they tend to pull/push each other. Remember, though, that some tyres are unidirectional, so cannot just be flipped round.

You may also find USA-required markings that rate the tyre for temperature (its ability to resist heat build-up), traction (grip) and tread wear. Temperature and traction are rated as A, B or C, in descending order: ideally you want to see A or B on your tyre. Tread wear is measured as a figure between 60 and 600, where 60 means the tyre will be worn out in a few thousand miles and 600 means it will last into the next millennium. Much still depends on how you drive...